In systems such as a TV conference system, a TV phone system, and the like in which a motion image signal is transmitted to a distant place, the image signal is compressed and coded making use of the line-to-line correlation and the interframe correlation of a video signal to effectively utilize a transmission path.
When an image signal is compressed and coded, it is coded so that a created bit stream has a predetermined bit rate. However, in a practical operation, it may necessary to change the bit rate of the bit stream for the convenience of the transmission path.
Further, when, for example, a transmitted image signal is edited by a broadcast station, since it is edited in a unit of second, the image information of a frame is preferably independent from the image information of other frame. Accordingly, Long GOP (Group of Picture), which has a large number of frames that constitute GOP as a group of frames having correlated information, and Short GOP, which has a small number of frames that constitute GOP and is transmitted at a high bit rate (18 to 50 Mbps) must be transformed alternately so that image quality is not deteriorated even if image information is transmitted at a low bit rate (for example, 3 to 9 Mbps).
A system will be explained using FIG. 1 which can edit frames by, for example, recoding a Long GOP stream data transmitted and received through a transmission path to All Intra Frame stream data as Short GOP.
Long GOP stream data suitable for transmission is transmitted through a transmission path 1.
After a transcoder 2 decodes an MPEG GOP stream data of supplied through the transmission path 1 once by a decoder 21, it codes the stream data by a coder 22 so that it is entirely made to All Intra Frame and outputs coded All Intra stream data (SDTI CP (Serial Data Transport Interface Contents Package)) to a frame editor 3 of an SDTI CP interface.
The stream data edited by the frame editor 3 is supplied to a transcoder 4. After the transcoder 4 decodes the All Intra stream data supplied thereto by a decoder 23 once, it codes the stream data by a coder 24 so that it is made to an MPEG Long GOP stream data and outputs it to a predetermined data transmission destination through the transmission path 1.
When image information is repeatedly coded and decoded as described above, if an coding parameter changes each time it is used in coding, the image information is deteriorated. To prevent the deterioration of the image information, Japanese Unexamined Patent Application Publication No. 2000-059788 discloses a technology that can suppress deterioration of an image deteriorated by recoding by the use of coding history information inserted into a user data area of a picture layer of a bit stream.
There will be explained, for example, a case in which the coding history information is used in a system which can transform MPEG Long GOP into Short GOP that can be subjected to frame edition with reference to FIGS. 2 and 3. Note that, in FIGS. 2 and 3, the portions that correspond to those in FIG. 1 are denoted by the same reference numerals, and the explanation thereof is appropriately omitted.
First, the case in which the history information is used will be explained with reference to FIG. 2.
That is, a transcoder 31 is supplied with MPEG Long GOP through a transmission path 1.
Since the MPEG Long GOP is composed of three types of pictures each having a different coding feature (I-picture, P-picture, and B-picture), video data obtained by decoding the MPEG Long GOP is composed of video data having the features of the I-picture, P-picture, and B-picture depending on a frame. Accordingly, when the video data is recoded by MPEG Long GOP, image deterioration may occur when the video data having the features of the I-picture, the P-picture, or the B-picture is coded by a different picture type. When, for example, video data arranged as the B-picture before it was decoded, which was more liable to be deformed than the I-picture and the P-picture, is coded as the I-picture, the pictures in the vicinity of the video data is coded by prediction using the I-picture having much deformation as a reference image.
When, for example, the transcoder 31 receives stream data, which was coded by other transcoder in the past, through the transmission path 1, the transcoder 31 decodes the MPEG Long GOP stream data supplied thereto by a decoder 41 once and then codes it by a decoder 42 so that it is made to All Intra Frame. At the time, to prevent image deterioration due to the recoding, the parameters used in the coding executed in the past, that is, the parameters such as a coding picture type, a quantized value, and the like of the coded stream supplied to the decoder 41 are added to the All Intra coded stream as SMPTE (Society of Motion Picture and Television Engineers) 328M History data and supplied to a frame editor 3.
The stream data edited by the frame editor 3 is supplied to a transcoder 32 again. The transcoder 32 decodes the All Intra stream data with History data supplied thereto by a decoder 43. A decoder 44 recodes the decoded stream data to Long GOP using necessary parameters such as the picture type, the quantized value, and the like included in decoded History data and outputs it to the transmission path 1.
Next, a case in which parameter information is used will be explained with reference to FIG. 3.
A transcoder 51 is supplied with MPEG Long GOP through a transmission path 1.
A decoder 61, which is supplied with MPEG Long GOP stream data, acquires coding parameters necessary when the data is decoded, and supplies decoded video data and the obtained coding parameters to a coder 62. The coder 62 transforms the video data into an All Intra coded stream using the coding parameters supplied thereto and supplies the coded stream to a frame editor 3.
The stream data that is edited to a frame by the frame editor 3 is supplied to the transcoder 52 again. A transcoder 52 decodes the stream data supplied thereto by a decoder 63. When decoding is executed, the decoder 63 acquires necessary coding parameters and supplies decoded video data and the obtained coding parameters to a coder 64. The coder 64 transforms the video data into a Long GOP coded stream using the coding parameters supplied thereto and outputs it to the transmission path 1.
It is possible to prevent image deterioration by executing coding by reusing past coding information (the parameters of a picture layer and a macro block layer such as picture types of coding executed in the past, movement vector, quantized values, and the like) using History data or the coding parameters as described above. However, a stream whose bit rate, image frame, chroma format, and the like are different from those of a previous stream may be used in place of previous stream or may be inserted into the previous stream by, for example, edition and the like. In this case, all the image data cannot be coded by reusing information as to previous coding using history or parameter information.